Place the mushroom mixture in a large pan and add the vegetable broth, bay leaf, allspice, thyme (place a couple of leaves aside) and the pepper grains. Boil for 2 approx. 2 hours on low heat in order to infuse the aroma.
Cover a sieve with a moist kitchen cloth. Sieve the broth and press down what remains in the cloth. Season the broth with sherry.
Rub the left-over champignons until clean. Cut in thin slices and add to the broth. Sprinkle the left-over thyme leaves over the broth.